This invention relates generally to fluid flow sensors and, more particularly, to fluid flow sensors that incorporate a thermistor.
Fluid flow sensors of this particular kind are useful in conjunction with implantable devices such as cardiac pacemakers. Such devices typically now have the capability of monitoring numerous physical parameters of a patient's cardiac and circulatory systems, so as to detect and evaluate certain physiological problems and conditions. In addition to blood flow rate, the parameters that are monitored can include, for example, pulse rate, body temperature, and blood pressure. Based on its evaluation of these parameters, the pacemaker can execute any of numerous functions so as to correct for the problem or condition.
Blood flow rate sensors of this kind typically have included a thermistor implanted within a blood vessel and exposed to the flow of blood. A driver circuit directs an electrical current through the thermistor, which heats the thermistor and thereby causes its resistance to vary accordingly. The flow of blood past the thermistor carries heat energy away, thus reducing the amount of temperature increase. The amount of temperature increase thus varies with flow rate, with a maximum amount of temperature increase occurring for a flow rate of zero. The driver circuit monitors the rate of change of the thermistor's resistance, e.g., by monitoring the voltage drop across the thermistor while a constant current is applied, and it estimates the blood's flow rate accordingly.
One drawback to such prior blood flow rate sensors is that changes in ambient temperature, i.e., blood temperature, can affect the accuracy of the flow rate measurement. The ambient temperature affects the amount of heat the blood draws away from the thermistor, thus causing the thermistor's temperature to vary not only with flow rate but also with the ambient temperature. It should, therefore, be appreciated that there is a need for a flow rate measuring apparatus that provides an accurate measure of flow rate without being affected by ambient temperature. The present invention satisfies this need.